The Moon most likely formed as the result of a big whack — a collision between Earth and another planet. But it may have taken its final form as the result of a big splat — a collision between the Moon itself and a smaller moon.

The side of the Moon that faces Earth is different from the hidden farside. It has a lower elevation and a thinner crust, and it has more of the dark volcanic plains known as the lunar seas. Just why that’s the case is a puzzler.

The Moon probably formed after a planet the size of Mars hit Earth, spewing rock and gas into space. The material coalesced to form the Moon. But simulations by scientists at the University of California-Santa Cruz show that the big whack could have created a second moon that was much smaller.

That’s not the first suggestion that the collision formed more than one moon. But these simulations show a different outcome: The second moon hit the lunar farside. The collision was gentle enough that it produced more of a splat than a whack, with debris spreading out across the surface. This material formed a thicker crust and rougher terrain — just as we see today.

The debate is far from over, but it’s one possible explanation for the Moon’s “two-faced” appearance.

And one of those faces — the lunar nearside — is in beautiful view tonight. It’s the Hunter’s Moon — the full Moon after the Harvest Moon in September. It’ll light up the sky all night long.